1. Field of the Invention
The present invention relates to a gas compressor operating method applied to a gas compressor in which gas fuel supply conditions change, such as a gas compressor for compressing gas fuel, employed in a low-calorie-gas-fired gas turbine that uses low-calorie gas, such as steelmaking byproduct gas (BFG), as the fuel, and to a gas turbine operated with this operating method.
2. Description of Related Art
A known gas turbine plant in the related art is operated using low-calorie gas as the fuel, such as steelmaking byproduct gas (blast furnace gas: BFG) generated in a large amount in a steelmaking process, that is, a low-calorie-gas-fired gas turbine (hereinafter referred to as “gas turbine”) operated using low-calorie gas as fuel. Such a gas turbine uses a gas compressor that compresses low-pressure gas fuel and supplies the compressed gas fuel to a combustor. Accordingly, to prevent surging in the gas compressor, with regards to “pressure ratio” on the vertical axis versus “degree of opening of variable stator vanes” on the horizontal axis, a “limit pressure ratio” for operation is provided, as indicated by the solid line in FIG. 1.
The limit pressure ratio is a value that defines the operational upper limit of the pressure ratio between an intake absolute pressure and a discharge absolute pressure of the gas compressor for preventing surging. Accordingly, to prevent surging in the gas compressor, the operation is controlled so as to ensure some allowance for the surging limit by adjusting the degree of opening of the variable stator vanes (intake-flow regulating mechanism) so that the limit pressure ratio is not exceeded.
That is, in FIG. 1, when the pressure ratio rises from point A and increases, the pressure ratio will coincide with the limit pressure ratio at point A′ unless the degree of opening of the variable stator vanes is changed, and thus, operation that makes the pressure ratio increase further becomes impossible. However, if the variable stator vanes are actuated in an opening direction as the pressure ratio increases from point A, the limit pressure ratio also increases, as indicated by point A to point B in FIG. 1, which increases the operating range of the pressure ratio in which the occurrence of surging can be prevented, thus generating some allowance for a safe operating range.
Increasing the degree of opening of the variable stator vanes as described above means that a gas turbine that receives the supply of gas fuel whose pressure is increased by the gas compressor performing a gas fuel bypass operation.
Furthermore, the gas turbine that receives the supply of gas fuel whose pressure is increased by the gas compressor is provided with an interlock that immediately halts the operation to protect the equipment when the operating pressure ratio has exceeded the limit pressure ratio.
A known example of the related art for preventing surging in a compressor is a compressor-surging prevention system for a carbon-dioxide-recovery power generating plant, as disclosed in Japanese Unexamined Patent Application, Publication No. 2000-337109. In this case, the fluctuating concentration ratio between water vapor and carbon dioxide is controlled by a flow regulating valve to prevent surging in a compressor that uses a gaseous mixture of water vapor and carbon dioxide as a working fluid.
Furthermore, the power output of a gas turbine power generation system that mainly uses blast furnace gas changes with a change in the calorific value of the blast furnace gas generated. Therefore, as disclosed in Japanese Unexamined Patent Application, Publication No. 2004-190633, for example, calorific value control is performed by adding a cooling gas or a heating gas depending on the result of measurement of the gas fuel calorific value; however, because unstable combustion or blow-off sometimes occurs due to a delay in response to sudden changes in calorific value, it is disclosed that the system is stabilized by rapid control for calculating the calorific value of the gas fuel in real time.
The foregoing low-calorie-gas-fired gas turbine sometimes shows changes in the temperature of gas fuel supplied to the gas compressor (intake gas temperature) or uses unstable gas fuel whose gas fuel composition changes significantly, as in the case where a plurality of gas fuels with different compositions are mixed.
If the gas fuel supply conditions, such the an intake gas temperature and the gas fuel composition described above, change significantly, the intake gas temperature (T), the gas constant (R), and the specific heat ratio (κ), which are values that influence the surging limit pressure ratio, also change. That is, changes in the gas fuel supply conditions, such as the intake gas temperature and the gas fuel composition, will change the characteristics of the limit pressure ratio for preventing surging in the gas compressor.
However, in the known methods for operating a gas turbine equipped with a gas compressor that compresses gas fuel whose supply conditions change and supplies the compressed gas fuel to a combustor and in which the gas compressor is provided with variable stator vanes, changes in the gas fuel supply conditions for the gas turbine control and the interlock are not considered. Accordingly, if the gas fuel supply conditions change significantly, decreasing the limit pressure ratio for surging, there is a possibility that surging in the gas compressor cannot be prevented, which, in the worst case, would cause damage to the equipment.
Given this background, for a gas turbine equipped with a gas compressor that compresses gas fuel whose supply conditions change and supplies the gas fuel to a combustor and in which the gas compressor is provided with variable stator vanes, it is desired to develop a gas turbine operating method that can reliably prevent surging by coping with changes in the gas fuel supply conditions.